As is the case with other eukaryotes such as animals and yeast, the rate of protein synthesis in plants is also usually controlled by the initial translation reaction (NPL 1).
The initiation of translation is one of the important control stages for plants to control gene expression in quick response to changes in the external environment. For example, translation of the majority of mRNAs to proteins is repressed under various stresses such as temperature, osmotic pressure, and anaerobic conditions (low-oxygen). However, the translation of mRNA is not entirely repressed. The translation of some mRNAs, i.e., protein synthesis, is maintained. Such knowledge suggests the presence of a gene expression control mechanism, which selectively and quickly synthesizes necessary proteins while repressing the majority of protein synthesis, at the translational level in response to external environmental stresses.
In recent years, attempts have been made to exhaustively analyze changes in the translational state of each mRNA induced by various environmental stresses. These attempts are to exhaustively understand the translational state of each gene by subjecting mRNA fractionated according to the degree of ribosomal binding to microarray analysis (NPL 2 to 5). Because of these attempts, the presence of numerous mRNA species whose translation is not repressed even under stress has become evident, and it has become clear that changes in the translational level in response to stress are not digitally determined to be repressed or not repressed depending on the mRNA species, but such changes occur in a continuous manner as a whole.
At the same time, a relationship between changes in the translational state caused by stress and the 5′ untranslated region (5′ UTR) has been suggested. For example, it has been reported from an analysis in which the 5′ UTR of corn Hsp 101 or ADH or Arabidopsis thaliana HSP 81-3 was linked to a reporter gene, that the 5′ UTR is important for the escape from stress-induced translational repression (NPL 6 to 8). However, a detailed mechanism thereof has not been clarified.
Based on the above-described relationship, which became clear by polysome/microarray analysis, between the translational state of each mRNA species and the features of the 5′ UTR of Arabidopsis thaliana plants under dehydration stress, Kawaguchi et al. made an attempt to search for an intrinsic factor of the 5′ UTR, which regulates translational control under stress, and reported that there is a correlation between the translational state under dehydration stress and the features of the 5′ UTR such as the length of the 5′ UTR and low GC content in the 5′ UTR. However, important sequence features have not been found even in this report, and Kawaguchi et al. themselves state that the length of 5′ UTR and the rate of GC content are not considered to be determining factors of the translational state under stress (NPL 4).